Apparatus for selectively plating lead frames

ABSTRACT

A novel plated lead frame strip and a novel method and apparatus for selectively plating the strip is described. The strip comprises a plurality of identical lead frame sections. Each of the lead frame sections comprises a plurality of leads. Each lead has a lead tip. In the center of each section and surrounded by the lead tips is a die-attach pad. The apparatus comprises in combination three plating stations. Each of the three stations is provided for plating a predetermined part of each of the lead frame sections. In the first station, a means is provided for plating the tips and the pad of a lead frame section with a first predetermined metallic plating -- e.g., a gold or copper strike. In the second station, a means is provided for masking the tips and plating the pad only with a second predetermined metallic plating -- e.g., a gold finish. In the third station, a means is provided for masking the pad and plating the tips with a third predetermined metallic plating -- e.g., silver. The stations are arranged in such a fashion that all three may be plating simultaneously with each one plating its respective part in the desired plating sequence.

BACKGROUND OF THE INVENTION

The present invention is directed to a method and apparatus for platinglead frames used in fabricating integrated circuit devices and the like.In the fabrication of solid state devices, the lead frames areoftentimes heated when bonding a chip to the die-attach pad to form agold-silicon eutechtic. When a lead frame is heated, the heat, dependingon the metal used, oxidizes the metal. To avoid the effects of oxidationof the metal, and, in particular, to avoid the creation of anelectrically insulating oxide on the metal surfaces, in the area ofelectrical contacts, it has been the practice to plate the electricalcontact areas of the lead frames with gold or silver or like metal.

Initially, to have good electrical contact surfaces and form agold-silicon eutectic, an entire lead frame or at least the entiresurface of one side of the lead frame, in particular the side on whichthe semiconductor chip is bonded, was plated with gold. There was noselective masking of any parts of the frame. This type of plating used agreat amount of gold.

Later, to conserve gold and thus reduce costs, as the cost of goldincreased, the entire surface of the frame on the side on which the chipis attached was plated with silver and the use of gold was restricted inthat it was selectively bonded to the die-attach pad alone in a two-stepprocess.

In a still further development, the practice was to selectively restrictthe plating of gold to the die-attach pad and to the lead tips so as toalso conserve silver and gold.

In summary, in accordance with the prior described practices, leadframes have been provided with a gold-plated die-attach pad for formingthe required gold-silicone eutectic and a silver or gold plated lead tiparea free of oxidation created during fabrication for providing goodelectrical contacts.

With the still ever increasing cost of gold, however, the amount of goldused in plating each lead frame becomes an appreciable percentage of itscost and is, if unnecessary, a needless waste of an important naturalresource.

It would seem a simple matter to plate the lead tips and die-attach areawith silver and, thereafter, to plate the die-attach area with gold in atwo-step process. However, it is found that when gold is plated oversilver, the gold combines with the silver to the extent that as much as66% more gold is required for forming a suitable gold-silicon eutecticthan is required in the absence of silver as substrate plating.

SUMMARY OF THE INVENTION

In view of the foregoing, the principal objects of the present inventionare a selectively plated lead frame, and an apparatus for and method ofplating the same.

In accordance with the above objects, there is provided a novel platingapparatus comprising three plating stations arranged in a seriesfashion. In the first station, a means is provided for plating a goldstrike or copper on the area of a lead frame consisting of thedie-attach pad and tips of the leads. In the second station, a means isprovided for plating a gold finish on the die-attach pad. In the thirdstation, a means is provided for masking the die-attach pad plated inthe second station and for plating silver on the tips of the leadssurrounding the masked die-attach pad.

The stations are arranged in a series fashion so that a continuous stripor series of lead frames may be sequenced in a step-like fashion throughthe stations such that the plating performed in each station is carriedout simultaneously.

In general, the means provided in each station for plating the leadframes comprises a plurality of fluid jets, means associated with eachof the jets for masking the lead frames so as to restrict the flow ofplating fluid to selected areas of the lead frames and means for causinga current to flow between the plating fluid and the lead frame so as toremove the metal from the fluid and deposit it on the frames.

By plating the lead frames with the apparatus described in the mannerdescribed, the plating is performed quickly and efficiently and, becausegold is not plated over silver on the die-attach pad, there is asignificant saving in money and a valuable natural resource.

DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from a reading of the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a view of a portion of a typical lead frame strip which can beplated in the apparatus and according to the method of the presentinvention.

FIG. 2 is an enlarged view of one of the lead frames of FIG. 1.

FIGS. 3A, 3B and 3C are diagrammatic representations of the lead tip anddie-attach pad areas of the lead frame of FIG. 2.

FIG. 4 is a perspective representation of three plating stationsaccording to the present invention.

FIG. 5 is an elevation view of a first one of the plating heads of thepresent invention.

FIG. 6 is a plan view of the head of FIG. 5.

FIG. 7 is a perspective view of the head of FIGS. 5 and 6.

FIG. 8 is a perspective view of a first masking member according to thepresent invention.

FIG. 9 is a bottom plan view of the member of FIG. 8.

FIG. 10 is a perspective view of a back-up member according to thepresent invention.

FIG. 11 is a bottom plan view of the member of FIG. 10.

FIG. 12 is an end view of the member of FIG. 9.

FIG. 13 is a plan view of a second plating head member according to thepresent invention.

FIG. 14 is a cross-sectional view taken along lines 14--14 in FIG. 13.

FIG. 15 is a perspective view of the head member of FIG. 13.

FIG. 16 is a perspective view of a second masking member according tothe present invention.

FIG. 17 is a second perspective view of the back-up member of FIGS. 10and 11.

FIG. 18 is a perspective view of a third plating head member accordingto the present invention.

FIG. 19 is a plan view of one of the fluid jetting means of the head ofFIG. 18.

FIG. 20 is a cross-sectional view taken along lines 20--20 in FIG. 19.

FIG. 21 is a perspective view of a third masking member according to thepresent invention; and

FIG. 22 is a third perspective view of the back-up member of FIGS. 10and 11.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, there is shown a section of a representativelead frame strip 1 comprising a plurality of lead frame sections 2, 3, 4and 5. All of the sections 2, 3, 4 and 5, each one of which is shownbetween a pair of parallel dashed lines, are identical and arefabricated and processed together with other sections in coils typicallycomprising thousands of such sections. The sections, or lead frames asthey are sometimes referred to herein, are supported together for thispurpose by a pair of relatively wide margins 6 and 7. In margins 6 and 7there is provided a plurality of sprocket holes 8. Holes 8 are used foradvancing the strip 1 through various stages of fabrication andprocessing, including the step-like plating processing described herein.

As seen more clearly in FIG. 2, in which there is shown an enlarged viewof section 3 of the strip 1 of FIG. 1, there is provided in each of thesections 2-5, a plurality of elongated, generally L-shaped leads 10.Each of the leads 10 is terminated at its interior end by a relativelynarrow tip portion 11. Extending from the tip portion 11 is an exteriorportion 12. The end of portion 12 is joined to an adjacent section --e.g., section 2 or 4 -- by a portion of the lead having a reducedcross-section. Between adjacent leads 10 and between the leads 10 andmargins 6 and 7, there are also provided supporting strips or wires 13.The wires 13 are employed to support the leads until they are embodiedin a finished device, at which time they are cut away to separate theleads. In the center of each of the sections 2-5 there is provided adie-attach pad 20. Pad 20 is supported between the margins 6 and 7 by apair of wire supporting members 21. In subsequent processing of thestrip 1, when individual integrated circuit devices are being made, thesupporting members 21 and margins 6 and 7, like wires 13, are also cutaway and the sections 2-5 are separated. Those processing steps,however, form no part of the present invention and, therefore, are notfurther described.

As shown in FIG. 2, there is identified an area enclosed by arectangular dashed line 30. Line 30 identifies the area, including thepad 20 and the tip portion of the ends of the leads 10. Dashed line 30represents the exterior boundary of the areas plated in accordance withthe present invention.

Referring to FIGS. 3A, 3B and 3C, there are shown three rectangularareas 40. Each of the areas 40 corresponds to the area of the lead framesection 3 bounded by the dashed line 30 in FIG. 2. In the interior ofeach of the areas 40 there is shown a smaller rectangular area 41. Eachof the areas 41 corresponds to the area of the die-attach pad 20 in FIG.2.

As will become apparent, plating according to the present invention isin a sequence of three steps. In the first step, as represented in FIG.3A, the areas of the lead frame section corresponding to the areas 40and 41 are plated with a gold or copper strike, as shown by the diagonallines extending across both of the areas. In the second step, asrepresented in FIG. 3B, the area 40 outside of the area 41 is masked andthe area 41 is plated with a gold finish. In the third step, asrepresented in FIG. 3C, the area 41 is masked and the area 40 outside ofthe area 41 is plated with silver.

While the selective plating of the areas 40 and 41 with gold and silveras described is important in the present invention, it is to beunderstood and is contemplated that in certain applications, metalsother than gold and silver may be employed and that, once understood,the sequence of the steps herein described can readily be interchangedin specific applications. In this regard, however, it is important torecall that the sequence as outlined herein for plating gold on thedie-attach pad and silver on the lead tips should be followed if the useof excessive gold, caused by the large amount of gold which combineswith silver as previously described, is to be avoided.

Referring to FIG. 4, there is provided three plating stations 50, 51 and52. The exterior appearance of each of the stations 50, 51 and 52 isquite similar. The interior components of each, however, are speciallyconstructed for performing, respectively, the first, second and thirdplating steps described above. Before describing each of the stations inparticular, however, it may be helpful to describe a few of the commonfeatures.

As shown in FIG. 4, the strip 1 is fed typically from a coil into oneend of the station 50. After passing through station 50 in the directionof the arrow to the right of station 50, it enters station 51. Afterpassing through station 51, the strip 1 passes through station 52.Thereafter the strip may be coiled or further processed. The movement ofthe strip 1 through the stations 50, 51 and 52 is provided by a step orsequencing dirve motor assembly of conventional construction (not shown)which is adapted for transporting the strip 1 very accurately by meansof a sprocket which engages the sprocket holes 8. Other means, ofcourse, may be used. The amount or distance strip 1 is transportedduring each sequencing step depends on the length of the individualstations 50-52. More precisely, if the stations 50-52 platesimultaneously, 10 sections of the strip 1 during each sequence, thenthe transporting motor assembly will transport strip 1 a distancecorresponding to the length of 10 frame sections during each sequence orstep.

Immediately to the right of station 50 and in sliding contact with strip1 is a sliding contact 53. Contact 53 is coupled in any suitable mannerto the negative pole of a d.c. potential source. Since strip 1 iscontinuous throughout the stations 50-52, contact 53 serves as thecathode electrode for each of the stations. While only a single cathodeelectrode is illustrated, it is intended that a plurality of suchelectrodes may be employed to reduce or in other respects modify andcontrol current distribution in the strip 1.

The plating fluid employed in each of the stations 50-52 is recirculatedthrough the stations. To capture the spent fluid, there is provided forthe stations 50-52, a plurality of fluid containers 54, 55 and 56,respectively. Coupled to each of the containers 54-56 is a fluid pump59. Pump 59 is provided for pumping fluid from the containers 54-56through a pair of pipes 57 and 58 back through the stations. In atypical embodiment, this is most easily achieved by supporting each ofthe stations inside its respective container as shown. Having beendescribed generally, the stations 50-52, beginning with station 50, willnow each be described in detail.

Referring to FIGS. 5-12, there is provided in station 50, a head member61. The head member 61, as are all of the non-electrical parts of thestations which come in contact with the plating fluid, is made of aplastic or other non-metallic material which is compatible with theplating fluid. In the base of head member 61, there is provided a fluidmanifold 62. Manifold 62 may consist of an enlarged fluid passagewaywhich may be provided in the member 61 as by drilling. Extending fromthe manifold 62 is a plurality of fluid jets 63. Each of the jets 63comprises a tubular member 64 which extends upwardly in the centerr of achamber 65 and is provided with a fluid passageway 66. Passageway 66extends through the center of the member 64 and is provided to be influid communication with the manifold 62. Extending from each of thechambers 65 is a fluid discharge port or outlet 67. In the top surfaceof the head member 61 is a pair of mask-aligning channels 68. Also inthe top surface of the head member 61 and in a slot of its own, is awire or the like 69. Wire 69 is provided to extend over the open end ofthe passageway 66 in each of the jets 63 and is adapted for coupling tothe positive pole of the d.c. source which is coupled to the cathodeelectrode 53.

Referring to FIGS. 8 and 9, there is provided a masking member 70. Inthe under-surface of masking member 70 there is provided a pair ofparallel ribs 71. Ribs 71 are adapted to fit in the slots 68 in the topof member 61 to facilitate aligning the member 70 longitudinally on themember 61 and prevent its lateral displacement with respect thereto. Thelongitudinal displacement of the member 70 is restricted by a back-upmember 80 described below. Intermediate the ribs 71 is a plurality ofrectangular apertures 72. The size of each of the apertures 72corresponds to the areas 40 described above with respect to FIGS. 3A, 3Band 3C and the area bounded by the dash lines 30 in FIG. 2. The positionof the apertures 72 in the masking member 70 is such that each one ofthe apertures is provided to be in registration with one of thepassageways 66 in one of the jets 63 in member 61 when the member 70 isinserted and aligned longitudinally in the slots 68. At opposite ends ofthe masking member 70, there is also provided a plurality of guidingmembers 85. Members 85 are provided for engaging the sides of the strip1 and aligning it laterally with respect to the apertures 72 such thatthe areas 40 on the strip are precisely over the apertures at the timeof plating. The material of which the member 70 is made is such as toform a fluid-tight seal with the strip 1 at the edges of the area 40.

Referring to FIGS. 9 and 10, there is shown the backup plate or member80. In the underside of member 80 there is provided a recessed portionin which is mounted an inflatable bladder 81. Coupled to bladder 81 isan air hose fitting 82. Fitting 82 is provided for coupling the bladder81 to a source of air under pressure. Along each side of the member 80and extending outwardly therefrom there is provided a plurality ofrelatively short pin-like members or the like 83. Similarly, along eachside of the head member 61 and extending outwardly therefrom there isprovided a corresponding number of relatively short pin-like members orthe like 84. Pin members 83 and 84 are provided for receiving elasticmembers, such as rubber bands or the like (not shown) for securing theback-up members 80 and the masking member 70 securely to the top of thehead member 61.

In use, the masking member 70 is positioned between the back-up member80 and the head member 61. With the bladder 81 initially deflated, thestrip 1 is inserted between the members 70 and 80 and aligned by meansof the guide members 85 over the apertures 72. When the strip 1 is thuspositioned, the bladder 81 is inflated for sealing the backside of thestrip and each of the areas 40 in each of the lead frame sectionsdescribed with respect to FIG. 3A, is exposed to a flow of plating fluidfrom one of the jet fluid passageways 66. The wire 69 is then coupled,as by a switch (not shown), to the positive pole of the d.c. sourcecoupled to the cathode electrode 53. When the wire 69 is connected tothe d.c. source, a current will flow in the fluid between the jet 63 andthe strip 1. This will cause a metal, such as gold, in the fluid, toplate out and adhere to the exposed surface of the strip -- in thiscase, the area 40. After a predetermined period of time, the currentflow is terminated and the bladder 81 is deflated. At all times duringthe flow of fluid from the jet 63, spent fluid from the strip 1 willfall into the chamber 63 and be discharged freely through the dischargeoutlet 67 and into the container 54 for recirculation by the pump 55through the station.

It may be noted, at this point, that the recirculation through thestations 50-52 need not be interrupted during sequencing of the strip 1from one station to another. This is because plating will occur onlywhen there is a completed electrical path between the d.c. potentialsource, the fluid and the strip. Thus, after deflation of the bladder81, the section of strip 1 plated in station 50 is free to be movedlongitudinally to the next station, station 51, without interrupting theflow of fluid in the station. This reduces the need for certain valves,simplifies controls and results in significant economies.

Referring to FIGS. 13-17, there is provided in station 51, a head member90 for coupling to the pump 59. In member 90 and extending substantiallyfor the length thereof, there is provided a fluid manifold 91. Extendingupwardly from manifold 91 and centrally located in a chamber 92, is aplurality of tubular members 93. Each of the tubular members 93 isprovided with a central passageway in which is fitted a metallic tubularmember 94. Member 94 is provided with a tapered end portion 95 and aflared or rolled lower end portion 96. End 95 is provided for forming anozzle or nozzle outlet. Rolled end 96 is formed in the vicinity of themanifold 91 for making electrical contact with a pair of wires 97. Wires97 are provided for coupling to the positive pole of a d.c. sourcehaving its negaitve pole coupled to the contact 53. It may be noted atthis point that, by using the metallic nozzle 94 connected to the wires97, there is avoided the placement of an interfering member in the fluidflow from the nozzle. Extending from the lower end of each of thechambers 92 and through the side of member 90, is a discharge port oroutlet 98. Outlet 98 is provided for freely discharging spent fluid intothe container 55. In the top surface of member 90 is a pair oflongitudinal slots 99 for aligning a masking member 101.

Slots 99 are provided for receiving a corresponding pair of ribs 100 inthe masking member 101. As with the ribs 71 in FIG. 8, ribs 100 areprovided on the underside of the masking member 101 for aligning themasking member laterally on the head member 90. Between the ribs 100 inmember 101 and extending through the member 101 is a plurality ofrectangularly-shaped apertures 102. Each of the apertures 102 isprovided to be placed in registration with one of the nozzles 94 whenthe masking member 101 is fitted on the member 90. The size of each ofthe apertures corresponds to the size of the die-attach pad 20 and thearea designated 41 in FIGS. 3A, 3B and 3C. On opposite ends of themember 101 there is a plurality of guide members 103. Members 103 areprovided for aligning the strip 1 laterally with respect to theapertures 102.

The material used to make the member 101 is such as to be able to form afluid-tight seal with the strip 1 around the die-attach pad so as torestrict the fluid from the nozzle 95 to the die-attach pad area.

In FIG. 17, there is shown a back-up member 105 with n an air inputnozzle 106. Member 105 is provided with an inflatable bladder (notshown). Along opposite sides of member 105 is a plurality of pins 107.Corresponding pins 108 are provided on the sides of the head member 90.Rubber bands or the like (not shown) are employed for coupling to thepins 107 and 108 for securing the member 105 securely to the head member90 with the masking member 101 fitted between the two. As described withrespect to the member 80 of FIG. 9, member 105 is provided forproviding, when its bladder is inflated, a fluid-tight seal on the uppersurface of the strip 1. The fluid-tight seal is provided for preventingloss of plating fluid through the lead frame as well as to immobilizethe strip 1 during plating of the die-attach pad area.

In use, station 51 is operated in substantially the same fashion asstation 50 with respect to the positioning and aligning of the strip 1in the station, the inflation of the baldder of the back-up member 105,the coupling of the wires 97 to a source of potential and the dischargeof fluid through outlets 98.

Referring to FIGS. 18-22, there is provided in station 52, a platinghead member 110. In the top of member 110 there is provided a pluralityof pairs of associated input and output chambers 111 and 120. Each ofthe chambers 111 and 120 is separated by an intermediate fluid-tightwall-like member 121. In each of the chambers 111 there is provided apair of fluid passageways 112 and 113. Passageways 112 and 113 areprovided in fluid communication with a fluid manifold (not shown) in thebase of member 110 below the chambers 111. Fluid is provided to themanifold under pressure by the pump 55 which is coupled to the fluidmanifold input port 114. In fluid communication with each of thechambers 120 and extending through the side of the member 110 is a fluiddischarge outlet 115. Outlet 115 is provided for freely dischargingfluid from the chambers 111 and 120 into the container 54.

In the center of each of the wall members 121 and extending upwardlytherefrom is a rectangularly-shaped pedestal 122 for supporting amasking member 123, such as a pad of silicone rubber. Masking member 123has a size at least equal to the area of the die-attach pad 20 of thestrip 1 for masking the die-attach pad area.

On opposite sides of the wall member 121 and extending across the mouthor top of chambers 111 and 120 there is provided a pair of wires 125 and126. Wires 125 and 126 extend substantially for the full length of thehead member 110 and are provided for coupling to the positive pole of ad.c. potential source having its negative pole coupled to the slidingcontact 53.

Referring to FIG. 21, there is provided a masking member 130. In thecenter of member 130, there is provided a plurality ofrectangularly-shaped apertures 131. The edges of the apertures 131correspond to the exterior edges of the areas 40 in FIGS. 3A, 3B and 3C.Extending from the underside of member 130 on opposite sides ofapertures 131 is a pair of parallel rib members 132. Rib members 132 areprovided for fitting in a corresponding pair of slots 133 in the uppersurface of member 110. At opposite ends of the member 130, is aplurality of guide members 134. Members 134, like members 85 and 103,are provided for guiding the strip 1 and aligning it laterally withrespect to the apertures 131.

Referring to FIG. 22, there is provided a back-up member 140. Member 140is identical to members 80 and 105 of FIGS. 10 and 17, respectively.

In the sides of member 140 and head member 110, for example, there isprovided a plurality of pins 141 and 142. Pins 141 and 142 secure, as byrubber bands and the like (not shown), the member 140 to the head member110. Also, member 140 is fitted with an inflatable bladder (not shown)which is operable for sealing the upper surface of the strip 1 aspreviously described with respect to members 80 and 105.

In use, after the strip 1 is positioned in the station 52, as describedwith respect to stations 50 and 51, with the masking member 123 securelymasking the die-attach pad 20, plating fluid will flow over thesurrounding lead tips. The path of fluid flow is from the fluidpassageways 112 and 113 into the chamber 111. Once the chamber 111 isfilled, the fluid flows over the top of the wall 121, against the strip1, down into the chamber 120 from which it is discharged into thecontainer 56 through the outlet 115. With the wires 125 and 126 coupledas by a switch (not shown) to the positive pole of a d.c. potentialsource coupled to the cathode electrode 53, a metal in the fluid willplate out on the strip in the area surrounding the pad 20.

A preferred embodiment of the invention has been described. It isintended, however, that various changes may be made with respect theretowithout departing from the spirit and scope of the invention. Forexample, various types of lead frames, as well as other types ofmetallic strips, may be plated, which might require changes in therelative positions of the apertures in the several masking membersand/or a change in the central masking pedestal and member 122 and 123of the station 52. Also, the number of plating jets in the stations maybe increased or decreased to accommodate a particular applicaton.Further, it is intended that various types and combinations of metalsother than those described may be plated with the apparatus andaccording to the method of the present invention. Accordingly, the scopeof the invention is intended not to be limited to the embodimentdescribed, but to be construed as broadly as the following claimspermit.

What is claimed is:
 1. An apparatus for selectively plating a lead framestrip, said strip including a plurality of lead frames, each of saidframes including a plurality of leads, each of said leads having a tip,and a die-attach pad, comprising:means for providing a first plating oneach of said lead frames, said first plating being a plating directly onand restricted to the surface of a predetermined portion of said tip ofeach of said leads and said die-attach pad, said first plating meanscomprising masking means for masking the surface of each of said leadframes about said tip of each of said leads; means for providing asecond plating on each of said lead frames, said second plating being aplating directly on said first plating and which is restricted to thearea of said die-attach pad, said second plating means comprising amasking means for masking the surface of each of said lead frames aboutsaid die-attach pad; and means for providing a third plating on each ofsaid lead frames, said third plating being a plating directly on saidfirst plating which is restricted to the area of said tip portion ofeach of said leads said third plating means comprising a first means formasking the surface of said lead frames about said tip of each of saidleads and a second means for masking said die-attach pad, each of saidplating means further comprising means for jetting a plating fluidagainst each of said lead frames and means for providing an electricfield in said fluid between said fluid and said lead frames, saidjetting means in said third plating means further comprising a pluralityof input fluid chambers and a plurality of corresponding output fluidchambers and; a wall member separating each of said input fluid chambersfrom its corresponding output fluid chamber, a predetermined number offluid passageways being associated with each of said input fluidchambers and being adapted to carry fluid into said input fluid chamber;said second masking means for masking said die-attach pad comprising apedestal means supported on said wall member and a fluid sealing membersupported on said pedestal means for sealing against said die-attachpad.
 2. A plating apparatus according to claim 1 wherein each of saidfirst, said second and said third plating means further comprises a leadframe back-up member, said back-up member including an inflatablebladder and means for coupling said bladder to a source of air underpressure whereby said bladder may be inflated for forming a fluid-tightseal with said lead frames and preventing a loss of plating fluidtherethrough, and further wherein said bladder, when inflated,immobilizes said lead frame against said masking means.